The present disclosure relates to fluid foils and mechanisms and methods for altering the shape of a fluid foil. Fluid foils are used for a variety of applications including lifting bodies and directional control surfaces for aircraft, lifting bodies and directional control surfaces for watercraft, keels on sailboats, turbine blades, propeller blades, helicopter rotors, and wind turbine blades.
Some common fluid foils are designed as primarily static structures, for example, current wind turbine blades. Other common fluid foil designs may include movable sections typically referred to as flaps, ailerons, and trim tabs, for example, a wing on an airliner. Static fluid foils are commonly rotated to adjust their pitch, but components of the fluid foils do not move with respect to one another.
Design principles for fluid foils are commonly used to construct fluid foils suited to particular applications and account for the opposing forces of lift and drag that are created when a fluid flows over a foil. A fluid foil is commonly designed to create lift, which often depends on the speed and/or density of a fluid flowing over the fluid foil. Typically, when the fluid speed over a foil is relatively slow, the foil is designed as a high lift foil by having a large camber and a large angle of attack. These high lift foils tend to induce a relatively large drag force. When the fluid speed increases, however, there is a reduced need for a high-lift foil, and an increased need for a low drag foil. Optimally, camber would be reduced and the angle of attack made smaller. Changing the lift and drag characteristics associated with a fluid foil requires altering the shape of the fluid foil and/or the angle of attack with respect to the moving fluid. Therefore, static fluid foils necessarily embody design trade-offs for lift and drag depending on the application and expected fluid densities and speeds in which the foil will be operated.
Fluid foils capable of changing their shape have been constructed for many years. The Wright brothers used cables to warp the shape of the wings to control the Wright Flyer as described in U.S. Pat. No. 821,393. Since then many mechanisms for changing the shape of a fluid foil have been created. Flaps, ailerons, and trim tabs are currently used on aircraft wings, are typically exposed to the flowing fluid, and are moved to alter the lift and drag characteristics of a wing. Moving current flaps, ailerons, and trim tabs requires complex mechanical systems, which adds to the weight and expense of fluid foils, and commonly creates gaps in their overall contours, which increases the induced drag forces created by the fluid flowing over the foil.
Exemplary mechanisms for altering the shape of a fluid foil without using flaps or ailerons are described in U.S. Pat. Nos. 4,247,066; 5,367,970; 5,839,700; 6,010,098; and 6,045,096. The present inventor has realized that drawbacks to such mechanisms for altering the shape of a fluid foil are that they are often complex, delicate, use expensive materials such as shape memory alloy wires like Nitenol wires, or include two or more such drawbacks.